In xerographic or electrostatographic printing, such as occurs in a copier or “laser printer,” an image is created with marking material on a sheet, such as a sheet of paper or a transparency slide. At one point in the electrostatographic printing process, the sheet is typically heated, in a final fusing step, to permanently affix the marking material thereto.
As the sheet passes through the various processing stations in the printing apparatus, a curl or bend is frequently induced therein. This curl or bend may be inherent to the sheet material due to the method of manufacture thereof, or the curl can be induced by the interaction of a sheet with the processing stations within the printer. The curling of the sheet causes problems of handling as the sheet is processed within the printer, frequently producing jams or misfeeds within the printer. Even if the curl is induced only toward the end of the printing process, having curled output sheets is well known as a customer dissatisfier.
In many xerographic printers, a decurling station is provided downstream of the fuser. Typically a decurling station includes a relatively soft roll (or a flexible belt entrained around two or more rollers) urged against a relatively hard roll, forming a nip. When a sheet passes through the nip, any curl inherent in the sheet is in effect bent in the opposite direction, toward the hard roll, yielding a flat sheet. Typically, the pressure between the hard roll and the soft roll in the nip is adjustable, such as by the manual turning of a knob within the machine, or with a motor-driven mechanism.